Even granting that a normative fact exists (e.g., that deception is morally wrong), awareness of that fact does not by itself rationally compel an agent to act accordingly.

Korsgaard’s case for constructivism parallels Kant’s argument for the autonomy of practical reason, as Rawls reconstructs it. It starts by objecting that substantive realism fails to respond to the skeptical challenge that there really are no reasons to be moral. This is because realism simply assumes the existence of objective standards for morality without offering a rational basis for them; hence the realist affirms what the skeptic denies. As a consequence, the realist also fails to show why